Aromatic carboxylic acids are highly useful organic compounds. They are useful as intermediates for the preparation of other organic compounds, and as monomers for the preparation of polymeric materials. In particular, the naphthalene dicarboxylic acids are used for preparing photographic chemicals and dyestuffs. Naphthalene dicarboxylic acids can be also be used to prepare a variety of polyester and polyamide compositions. 2,6-naphthalene dicarboxylic acid (hereafter referred to as 2,6-NDA) is a particularly useful aromatic carboxylic acid which can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate). Polyesters prepared from 2,6-NDA have excellent heat resistance, gas barrier, and mechanical properties. Therefore, much research in the art has focused on methods of preparing 2,6-NDA. Most of the methods for making 2,6-NDA involve numerous steps, many of which address purification of the product.
It is known in the art to produce the crude dialkali salt of naphthalene-2,6-dicarboxylic acid by heating alkali naphthalene-.alpha.-monocarboxylate(alkali 1-naphthoate), alkali naphthalene-.beta.-monocarboxylate(alkali 2-naphthoate), or mixtures thereof at high temperatures in carbon dioxide or other gaseous atmosphere, (German Pat. No. 953,072) or by similarly heating dialkali naphthalene 1,8 dicarboxylate (German Pat. Nos. 932,125 and 1,002,316). This reaction is usually referred to as a Henkel disproportionation reaction and produces a product consisting mainly of a single thermodynamically favored disalt product. It is known in the art to prepare the alkali salts used in this reaction by heating naphthalene .alpha.-monocarboxylic acid, naphthalene .beta.-monocarboxylic acid, or naphthalene 1,8-dicarboxylic acid, with hydroxides, carbonates, bicarbonates, etc. of alkali metals in an aqueous medium under heating. When potassium naphthoate is the feed in a Henkel disproportionation reaction, the product is the salt of the desired 2,6-NDA, 2,6-K2NDA. In addition to the desired product, this reaction also typically produces unreacted potassium naphthoate and 2,3 K2NDA. (In describing the invention certain abbreviations will be used which have the following meanings: KNA=potassium naphthoate; 2,6-K2NDA=dipotassium salt of 2,6-naphthalene dicarboxylic acid; and 2,6-NDA=2,6-naphthalene dicarboxylic acid.)
Methods of separating the desired disalts of 2,6-naphthalene dicarboxylic acid are known. After the Henkel disproportionation reaction, the total reactor effluent is generally dissolved in water to separate out the aromatic core molecule produced by the Henkel disproportionation and the catalyst as separate phases with the unconverted feed and the desired reaction product dissolved in the aqueous phase. In one method known in the art, the disproportionation reaction product, containing the dialkali salt of NDA and the salt of naphthalene mono- or di-carboxylic acid, is dissolved in water, and the water insoluble matters are filtered off. Then a mineral acid is added to the system to release naphthalene carboxylic acids. The free acids are recovered from the system by filtration and are further boiled with an organic solvent such as a lower aliphatic carboxylic acid to separate 2,6-NDA which is insoluble in the organic solvent, from the free dicarboxylic acids which have been separated from the aforementioned unreacted products and side products upon addition of a mineral acid, and are soluble in organic solvent. A disadvantage of this method is that the side product alkali salt of a mineral acid cannot be directly re-used in the preparation of starting material for the disproportionation reaction. (See U.S. Pat. Nos. 2,823,231 and 3,671,578.)
CA 864587 discloses a method for separating 2,6-NDA by disproportionating the alkali salt of 2,6-NDA by heating it in water or water-containing organic solvent to form free 2,6-NDA and by-product dialkali salt, where the former is precipitated.
One method of purifying 2,6-K2NDA to acceptable levels of potassium naphthoate is by performing two successive crystallizations on the crude 2,6-K2NDA from the disproportionation reaction. This process is energy intensive because the water solubility of 2,6-K2NDA varies only weakly with temperature and the product must be recovered by an evaporative crystallization step which involves evaporating about half of the water charged to the crystallizer. Furthermore, laboratory experiments have shown that the crystallization process cannot recover 2,6-K2NDA in high yield from the crystallization when larger than normal levels of impurities are present in the crystallization feed due to co-precipitation of the impurities with the product 2,6-K2NDA.
None of the references in the art pertaining to production and purification of 2,6-NDA teach the use of activated carbon for separating unreacted feed from product aromatic dicarboxylic acids. U.S. Pat. No. 5,770,764 does disclose the use of carbon beds to remove impurities. In '764 the 2,6-NDA is made by direct oxidation and the carbon bed is used to remove impurities such as trimellitic acid, bromo-2,6-NDA, 2-formyl-6-naphthoic acid, 2 naphthoic acid, other impurities and cobalt and manganese catalyst components.
It is known in the art to use cyclic adsorption to purify petrochemical products. For example, U.S. Pat. No. 3,069,470 discloses the use of type X zeolites for the separation of the meta isomer from other isomers of toluidine.
From U.S. Pat. No. 4,480,129 it is known that X and Y type zeolites, exchanged with transition metals, are paraselective in a mixture of isomers of toluidine.
U.S. Pat. No. 4,642,397 discloses that an adsorbent comprising an X or Y type zeolite cation exchanged with a cation selected from the group K, Na, Ca, Ba, Li, or Mg can be employed to selectively separate 2,4-dinitrotoluene from a feed mixture comprising 2,4-dinitrotoluene and at least one other isomer.
In U.S. Pat. Nos. 5,622,682 and 5,779,998 adsorbents are employed to recover halocarbons from a gas mixture.
Where activated carbon has been used in the art, it is typically used to adsorb trace levels of materials from a solution, rather than percent level impurities in the presence of in the range of ten times excess of a very similar molecule.
Known methods of separating unreacted feed components from 2,6-K2NDA require a number of steps. The prior art for 2,6-NDA purification made by a disproportionation reaction is crystallization or precipitation with carbon dioxide and often the potassium naphthoate is separated in a form which is not recyclable. Therefore, there is a need in the art for a simpler process for separating unreacted feed such as potassium naphthoate from 2,6-K2NDA. This would be a distinct advance in the art relating to purification of 2,6-NDA.
The present invention provides such a process. Other objects and advantages of the invention will be apparent to those skilled in the art from the following detailed description and appended claims.